(1) Field of the Invention
The present invention relates to a method of making polycide-to-polycide contacts for multilevel interconnections on semiconductor integrated circuits, and more particularly relates to a method and structure for forming low-resistance contacts between interconnecting polycide layers.
(2) Description of the Prior Art
Advances in semiconductor processing technologies, such as advances in high-resolution photolithography and anisotropic plasma etching, continue to reduce the feature sizes of semiconductor devices. These reduced feature sizes include smaller contact openings leading to higher contact resistance. For example, current contact opening feature sizes are now typically less than 0.5 micrometer (um). The increase in this parasitic resistance in series with circuit devices, such as field effect transistors (FETs), degrade the circuit performance and therefore is undesirable. A further concern is wide distribution in contact resistance (R.sub.c), which is also undesirable.
It is common practice in the semiconductor industry to interconnect the semiconductor devices by using multilayers of patterned heavily doped polysilicon/silicide layers, commonly referred to as polycide layers, and by metal layers to form the integrated circuits. Interlevel dielectric (ILD) layers, hereafter referred to as "insulating layers," having contact openings (via holes) between the polycide layers are used to electrically insulate the various polycide and metal layers on the substrate. On Ultra Large Scale Integration (ULSI) the number of contacts now well exceeds a million and it is important to have consistently low contact resistance (R.sub.c) from contact to contact.
One method of providing consistently low contact resistance between aluminum metal levels, by the prior art, is to in-situ sputter clean the contact openings just prior to physical vapor depositing the next level of aluminum to avoid the aluminum oxide formation that readily forms during exposure to the ambient. Unfortunately, this requires retrofitting the deposition tool with a sputtering system and further complicates the process.
High contact resistance problems are also a concern between silicide/polysilicon (polycide) to silicide/polysilicon (polycide) layers that are also typically used to form part of semiconductor devices such as FET gate electrodes, word lines, bit lines, etc. on integrated circuits, such as dynamic random access memory (DRAM), static random access memory (SRAM), and microprocessors. A typical prior art contact between two tungsten silicide/polysilicon patterned layers is shown in FIG. 1. The contact structure is shown built on a semiconductor substrate 10 having an insulating layer 12 thereon. Shown is a first polycide layer composed of a first polysilicon layer 14 and a first silicide layer 16 comprised, in part, by the gate electrodes for an FET. The first polycide layer is then electrically insulated by depositing an insulating layer 20, for example, by a chemical vapor deposited (CVD) oxide. The contact openings 4 are then formed in the insulating layer 20 to the surface of the first silicide layer 16 by using conventional photolithographic techniques and anisotropic plasma etching. Still referring to FIG. 1, an undoped second polysilicon layer 21 is deposited over the insulating layer 20, and in the contact openings for making contact with the top surface of the first silicide layer 16. A doped polysilicon layer 24 is deposited on the undoped polysilicon layer 21. The undoped polysilicon layer 21 is used to prevent diffusion of dopants from polysilicon layer 24 into the substrate 10 when the contact is made over the cell contact regions (N.sup.-), such as FET source/drain regions, to prevent deep junctions from forming. The second level of interconnecting metallurgies is now completed by depositing a second silicide layer 26. Layers 26, 24, and 21 are then patterned using conventional photolithographic and plasma etching to form the second patterned conducting layer.
Unfortunately, it is difficult to form consistently low contact resistance for the contacts, as in FIG. 1, because of the difficulty in removing residual polymers that occur during the etching of the contact openings, such as opening 4 in FIG. 1, and the subsequent removal of the photoresist contact mask. For example, contacts having minimum feature sizes of 0.5 um or less can have contacts with resistance that varies from as low as 100 ohms to values exceeding 2000 ohms. Furthermore, interface treatments, such as plasma etching in a gas mixture containing CH.sub.4 and O.sub.2 to treat the tungsten silicide surface, are not effective, even when portions of the top surface of the silicide layer 16 are partially removed. Alternatively, implant doping of the tungsten silicide layer 16 in the contact openings does not provide consistently low contact resistance.
Therefore, there is still a strong need in the semiconductor industry to further provide methods for forming contacts with low contact resistance for these interconnecting metallurgies, while eliminating the need for additional processing steps to reduce the resistance, and thereby providing a more cost-effective manufacturing process.